1. Field of the Invention
This invention relates generally to amplifying circuits and, more particularly, to amplifying circuits, referred to as isolation amplifier circuits, for which an input signal must be electrically isolated from an output signal.
2. Discussion of the Related Art
An isolation amplifier circuit is used to isolate a dc component of an input signal from a dc component of an output signal. Two general types of isolation amplifier circuits are currently in use. In one of the types of amplifier circuits, the input signal is electrically isolated from the output signal by means of a transformer circuit. The transformer circuit is popular because of the well-known design characteristics. However, because of the unacceptable low frequency and DC components, the transformer provides only limited usefulness in the isolation amplifier circuit application. This limitation can be overcome by modulation of the input signal applied to the input terminals of the transformer and reconstruction of the output signal at the output terminals of the transformer. This procedure provides a low frequency compensation, but at the expense of degradation of high frequency performance. While complex circuits can be devised to provide appropriate response in both frequency ranges, the result becomes a circuit of increasing complexity and expense. A second common technique is the use of an optical coupler by which a light-emitting diode produces electromagnetic radiation. The radiation is then detected by a photosensitve diode. The optical coupling apparatus is relatively expensive and typically requires a power supply to activate the light-emitting diode of the input circuit.
A need has therefore been felt for a isolation amplifier circuit which would be responsive to the low frequency signals and which would still provide complete electrical isolation between the input signal and the output signal.